Typical cementing assemblies involve a landing collar above a float collar, which is in turn above a float shoe. The float shoe and float collar have valves in them and generally have fairly small openings for such valves. These components are located at the bottom of a liner string to be cemented with some tubulars mounted further below. The assembly at the bottom of the string, when inserted into a highly deviated wellbore, has encountered some operational problems with the cement seal. The "shoe track" refers to the termination of the wellbore where these components are inserted prior to the pumping of cement. When the shoe track is highly deviated and cement is pumped to displace the drilling mud, formation fluid leakage through the shoe track has been experienced. The shoe track generally consists of approximately 100 ft. of liner below the float shoe, which by design remains full of cement after a wiper plug is pumped down against the landing collar to displace the pumped cement out of the liner being cemented down to the landing collar. With the shoe track highly deviated, cement has a tendency to allow water migration if the cement is prepared at the surface with an excess of water. With the shoe track essentially horizontal or close to it, i.e., greater than about 70.degree. deviation from vertical, migration of water upwardly creates a flow channel through the cement in the shoe track, allowing formation fluids to migrate into the casing or fluid loss into the formation. One of the reasons excess water is used with the cement mixture is that the cement preparations on the surface are frequently made "on the fly" as opposed to carefully measured batch operations. The control system for on-the-fly mixing of cement slurries is not that precise, resulting in periodic blends which have an excess of water.
Another phenomenon that could lead to undermining the cement seal in the shoe track is the phenomenon of shrinkage of the cement. Generally, cement when setting will shrink approximately a half of a percent. Shrinkage, especially in the area of the float shoe or float collar, could result in movement of these valves off their seats to leave them in a slightly open position. Thus, the ability of the float shoe and float collar to act as seals after the cement has set can be adversely affected.
The shrinkage of the cement also presents opportunities in highly deviated wellbores for fluid flow adjacent the inside wall of the casing.
A phenomenon known as "roping" has been suspected as the cause of inefficient displacement of mud with the cement. One suspected cause is the necessity of pumping the cement through the float shoe and float collar which have fairly small bores. The flow pattern of the cement emerging from the float shoe and float collar is such that it leaves pockets of mud trapped which are not displaced by the cement. These pockets of mud then can become the source of future leakage problems through the cement job through the shoe track.
It is thus an objective of the present invention to provide an economical device that can be installed in the liner assembly, particularly for deviated wellbores with an inclination angle of more than about 70.degree. from the vertical so that leakage paths through the cement can be eliminated. The device should be effective against the roping phenomenon as well as provide seals to potential leakpaths adjacent the inside wall of the casing. The device should also be sufficiently soft so that it can be readily drilled out if necessary. Due to the harsh environment, a device which can accomplish these objectives while being static, i.e., with no moving parts, is desirable. These objectives have been addressed in the present invention, a preferred embodiment of which is described in more detail below.